Method for age-dating oil is developed

STANFORD -- The Jurassic (180 million to 140 million years ago) was a very
good age for oil formation. So too was the Cretaceous (140 million to 65
million years ago). But, until now, oil industry experts have lacked a direct
way to date their crude.

That may change, however, with the development of a biological marker
system developed by a joint academic-industry research team headed by J.
Michael Moldowan, a research professor at Stanford University. The new dating
method is described in the August 5 issue of the journal Science. Other
members of the team are Jeremy Dahl from the Chevron Petroleum Technology
Company, Bradley J. Huizinga from ARCO International Oil & Gas Company,
Stanford science and engineering technician Frederick J. Fago, Leo J. Hickey
from Yale University, Torren M. Peakman from the University of Bristol,
England, and David W. Taylor from Indiana University-Southeast.

Their dating system measures the relative amount of an organic compound,
oleanane, in the oil. Oleanane is highly associated with the angiosperms,
flowering plants that have evolved and spread since the early Cretaceous.
Many flowering plants produce derivatives of oleanane, called oleanoids, that
are toxic to predators.

Because angiosperms become more plentiful as time progresses, younger
organic deposits that are converted into oil are likely to contain more
angiosperms. Because the geological processes involved convert the oleanoids
in the angiosperms into oleanane, the more oleanane found in a given oil
sample, the more recent it is likely to be.

The method is not a very precise yardstick. Oils of any age can lack
oleanane if flowering plants were not part of the material from which it
formed. But lack of oleanane is a significant clue that the oil may have
formed in the Jurassic or older times, before angiosperms evolved. If the
compound is present in relatively small amounts, the crude is almost
certainly Cretaceous or younger. If it contains large amounts of the organic
substance, on the other hand, its pedigree most likely dates from the
post-Cretaceous or Tertiary Age (65 million to 5 million years ago).

But "even this crude level of dating can be very useful in petroleum
exploration," said Moldowan, who recently moved to Stanford from Chevron.

Dating the oil in a given reservoir provides important clues to its source
formation, something that often is not obvious because oil can migrate large
distances underground. Therefore, dating can help petroleum geologists
determine the specific "plays" - geological environments - on which they
should concentrate their efforts, Moldowan said.

To calibrate this scale, the scientists first measured the amount of
oleanane in 103 samples of petroleum source rock that could be dated by
conventional means.

The researchers knew that they could not use the absolute concentrations
of oleanane because the amounts of this compound can vary by a factor of a
thousand or more, depending on how much heat a given sample has undergone.
They have attempted to correct for this problem by using the ratio of the
oleanane concentration relative to a very similar chemical, called hopane,
that is ubiquitous in crude oil and whose precursors are produced by
bacteria. Because hopane should degrade at about the same rate as oleanane,
the ratio of the two should be relatively independent of the petroleum's
thermal history, the researchers said.

When they obtained these ratios and compared them with the age of their
samples and the number of families of angiosperms in the fossil record, they
found that, although there were some differences, variations in the level of
the biological marker are broadly consistent with the fossil record. These
differences support Taylor and Hickey's hypothesis that the ancestral
angiosperm were herbaceous.

The researchers did get a surprise, however, when they analyzed a
307-million-year-old concretion associated with Pennsylvanian Age coal seams
in Illinois. Despite the fact that this "coal ball" dates to well before the
period when flowering plants first came on the scene, they found measurable
levels of oleanane.

"It must be that specific piece of coal," Moldowan said. "To the best of
my knowledge, oleanane has never been found in a sample this old before. That
could mean that we are seeing oleanane being produced by the ancestors of the
angiosperms. This lineage has recently been referred to as
'stem-angiophytes.' "

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